The present invention relates to fiber Raman amplifiers and, more particularly, to the use of monitoring information on an applied Raman pump signal to analyze the performance of the amplifier.
Optical amplification by stimulated Raman scattering in a single mode fiber is of interest for applications in optical communication systems. In particular, the use of Raman amplification in wavelength division multiplexed (WDM) optical communication systems is particularly attractive, since the bandwidth of the Raman amplifier can be large enough to accommodate two or more WDM channels. The performance of intensity-modulated WDM systems using Raman amplifiers, however, may be limited by the cross talk between the information channels and the amount of power actually coupled from the Raman pump into the information channels (as well as fiber nonlinearity-related penalties).
The cross talk in Raman amplifiers is mediated by the pump source. That is, each modulated channel causes pattern-dependent pump depletion that is subsequently superimposed on all other channels during the amplification process. The cross talk has been found to be dependent on the modulation frequency of the channels, as well as the relative speed between the channels and the pump. For this reason, the amount of cross talk present in co- and counter-propagating pump configurations has been found to differ significantly.
Regarding the issue of pattern dependence, if one channel is transmitting a long stream of xe2x80x9c1 ""sxe2x80x9d or xe2x80x9c0""sxe2x80x9d, the power in the adjacent channel will change. Thus, optical amplifiers would perform best in situations where the data pattern is random. The pattern-dependence problem, which leads to xe2x80x9cpower stealingxe2x80x9d between channels has been addressed in co-pending application Ser. No. 09/990,206, also filed by the applicant on Nov. 21, 2001, which discloses the use of a radio frequency (RF) modulation signal impressed on a conventional continuous wave (CW) pump signal. In our co-pending application, the modulation frequency and depth are controlled to introduce a slight fluctuation to the input power level of the pump signal, resulting in introducing a sufficient degree of randomness to overcome the cross talk problem, and increase the amount of power coupled from the Raman pump into the information-bearing signals. Modifying the modulation index of the RF signal applied to the pump has been found to control any non-linearities present in the amplified output signal.
While this arrangement is helpful, a need remains to monitor the performance of the fiber Raman amplifier system. Existing methods in the prior art may use a separate communication channel as a xe2x80x9cmonitoringxe2x80x9d channel, observing and analyzing a transmitted monitoring signal at a receiver. Although useful, this method decreases the efficiency of the system by requiring the dedication of a channel to the monitoring process. Another method utilizes xe2x80x9cmonitoring tonesxe2x80x9d impressed on a transmitted data signal. This method results in introducing a penalty in terms of the recovered information from this data signal.
Thus, a need remains in the art for an arrangement which can monitor the performance of a fiber Raman amplifier without incurring penalties on the transmitted data signals.
The need remaining in the art is addressed by the present invention, which relates to fiber Raman amplifiers and, more particularly, to the use of monitoring information on an applied pump signal to analyze the performance of the amplifier.
In accordance with the present invention, the RF signal used to modulate the pump is itself modulated with a low data rate xe2x80x9csignaturexe2x80x9d signal, chosen to be unique for that particular pump. At the receiver end, a filter and demodulator are used to separate the pump from the amplified information signal and recover the unique signature signal. Problems with a given pump source can be presumed, therefore, if its associated, recovered unique signature signal is absent, exhibits a low signal-to-noise ratio (SNR), or an excessive bit error rate (BER). In systems which utilize multiple pump sources, the use of a unique signature signal for each pump source allows for the receiver to distinguish between the pumps and monitor each source.
In an alternative embodiment of the present invention, the unique signature may comprise a frequency offset applied to the RF modulation signal at the pump source, where this offset is then be used at the receiver to identify each separate pump source. Once detected, an appropriate mechanism can be utilized to correct the defective pump source (e.g., replace optic device, increase power, etc.).